#include<iostream>
#include<cstdio>
using namespace std;

int main()
{
	int q = 2, p = 1;
	int n = 0;
	cin >> n;
	double sum = 0.0;
	for (int i = 1; i <= n; i++)
	{
		sum += q * 1.0 / p;
		q = q + p; 
		p = q - p;
	}
	printf("%.4lf", sum);
	
	return 0;
}
